(a) Transverse cross section in the xy-plane of the PC made of a triangular array of solid cylinders surrounded with air. Thin arrows indicate the directions of propagation ΓJ and ΓX. (b) Dispersion curves of the PC. The ABG and the band gap for normal incidence (BG0°) are shown in gray, and the negative refraction band is shown by the white and gray checkerboard area. Inset: full and reduced Brillouin zones for the triangular lattice, showing the definition of the labels for the symmetry points (see text for details). The dashed line represent the dispersion curve in the air, i.e. when no dispersion occurs (, with f the frequency, k the wave number, and c_air the speed of sound in the air), along both principal directions of propagation ΓJ and ΓX.